1. Field
An embodiment of the present invention relates to an optical unit, an illumination optical apparatus, an exposure apparatus, and a device manufacturing method.
2. Description of the Related Art
In a typical exposure apparatus of this type, a light beam emitted from a light source travels through a fly's eye lens as an optical integrator to form a secondary light source (a predetermined light intensity distribution on an illumination pupil in general) as a substantial surface illuminant consisting of a large number of light sources. The light intensity distribution on the illumination pupil will be referred to hereinafter as “illumination pupil luminance distribution.” The illumination pupil is defined as a position such that an illumination target surface becomes a Fourier transform surface of the illumination pupil by action of an optical system between the illumination pupil and the illumination target surface (a mask or a wafer in the case of the exposure apparatus).
Beams from the secondary light source are condensed by a condenser lens to supposedly illuminate the mask on which a predetermined pattern is formed. Light passing through the mask travels through a projection optical system to be focused on the wafer, whereby the mask pattern is projected (or transferred) onto the wafer to effect exposure thereof. Since the pattern formed on the mask is a highly integrated one, an even illuminance distribution must be obtained on the wafer in order to accurately transfer this fine pattern onto the wafer.
There is a conventionally proposed illumination optical apparatus capable of continuously changing the illumination pupil luminance distribution (and, therefore, the illumination condition) without use of a zoom optical system (cf. Japanese Patent Application Laid-open No. 2002-353105). The illumination optical apparatus disclosed in the Application Laid-open No. 2002-353105 uses a movable multi-mirror composed of a large number of micro mirror elements which are arranged in an array form and angles and directions of inclination of which are individually drive-controlled, and is so configured that an incident beam is divided into beams of small units corresponding to reflecting surfaces of the mirror elements, the beams of small units are folded by the multi-minor to convert a cross section of the incident beam into a desired shape or a desired size, and, in turn, a desired illumination pupil luminance distribution is realized.